Alternator Voltage Drop Test
Voltage drops in the charging circuit can be very difficult to find and cause you many hours of frustration. We hope that the following diagrams will help you troubleshoot the charging system.
Some key points to remember are:
- Any added resistance to the Charging circuit can result in decreased alternator performance. For example, with a system charging 100 amps, when we add 0.01ohms resistance to the B+ or ground circuit, we will reduce the voltage in the system by 1 volt. This will lower overall system performance
and reduce the life of the battery and charging system.
- When performing voltage drops always have your positive lead of the multimeter on the connection closest to the alternator.
- When performing voltage drop tests current must be flowing in the circuit. This means we must have the vehicle running, alternator charging and accessories turned on.
Step 1. Alternator Voltage Drop - Positive Circut
With positive lead of the multimeter touching the positive post on the alternator and the negative lead of multimeter touching the positive (+) post (not the cable end) of the battery and the engine running apply a load to the system.
Watching the voltmeter (wired as shown above), the reading on voltmeter should be less than .2 volts.
If the reading is more than .2 volts go to step 2. In step 2 we will check the components of the positive (+) circuit.
Step 2. Alternator Voltage Drop - Positive Circuit Components
With the positive lead of the multimeter touching the point closest to the alternator and the negative lead of multimeter touching the point to be measured (remember if measuring to a post, measure the post not the cable end) and with the engine running apply a load to the system.
Watching the voltmeter (wired as shown above), the reading on voltmeter should be less than .2 volts.
Check each component and link in the circuit. Those components with a voltmeter reading of greater than .2 volts should be repaired or replaced. Check the component or link to verify that the voltage drop is now .2 volts or less. Check across every connection, cable and component.
Step 3. Alternator Voltage Drop - Ground Circuit
With positive lead of the multimeter touching the negative (-) battery post (not the cable end) and the negative lead of the voltmeter touching the housing of the alternator and with engine running apply a load to the system.
Watch the voltmeter (wired as shown above), the reading on voltmeter should be less than .2 volts.
If the reading is more than .2 volts check components in the ground circuit. Check components just like we did in step 2 but place the positive lead from multimeter to the connection closest to the negative battery post.